This invention relates generally to a meltblowing die system. In one aspect the invention relates to a meltblowing die comprising a plurality of self-contained, interchangeable modular units. In another aspect, the invention relates to a meltblowing die for meltblowing polymer onto a substrate or collector wherein the deposition pattern is wider than the effective length of the die. In still another embodiment, the present invention relates to a modular meltblowing die wherein adhesive is deposited uniformly across a substrate.
Meltblowing is a process in which high velocity hot air (normally referred to as xe2x80x9cprimary airxe2x80x9d) is used to blow molten fibers extruded from a die onto a collector to form a web, or onto a substrate to form a coating or composite. The process employs a die provided with (a) a plurality of openings (e.g. orifices) formed in the apex of a triangular shaped die tip and (b) flanking air passages. As extruded rows of the polymer melt emerge from the openings, the converging high velocity air from the air passages contacts the filaments and by drag forces stretches and draws them down forming microsized filaments. The microsized filaments are deposited in a random or uniform pattern on a collector or substrate.
In some meltblowing dies, the openings are in the form of slots. Generally, however, the die openings are in the form of orifices. In either design, the die tips are adapted to form a row of filaments which upon contact with the converging sheets of air are carried to and deposited on a collector or a substrate in a random or uniform manner.
Meltblowing technology was originally developed for producing nonwoven fabrics but recently has been utilized in the meltblowing of adhesives onto substrates. In meltblowing adhesives, the filaments are drawn down to their final diameter of 5 to 200 microns, preferably 10 to 100 microns, and are deposited at random or uniformly on a substrate to form an adhesive layer thereon onto which may be laminated another layer such as film or other types of materials or fabrics.
In the meltblowing of polymers to form nonwoven fabrics (e.g. webs), the polymers, such as polyolefin, particularly polypropylene, are extruded as filaments and drawn down to an average fiber diameter of 0.5 to 10 microns and deposited at random on a collector to form a nonwoven fabric. The integrity of the nonwoven fabric is achieved by fiber entanglement with some fiber-to-fiber fusion. The nonwoven fabrics have many uses including oil wipes, surgical gowns, masks, filters, etc.
The filaments extruded from the meltblowing die may be continuous or discontinuous. For the purpose of the present invention, the term xe2x80x9cfilamentxe2x80x9d is used interchangeably with the term xe2x80x9cfiberxe2x80x9d and refers to both continuous and discontinuous strands.
The meltblowing process grew out of laboratory research by the Navel Research Laboratory which was published in Navel Research Laboratory Report 4364 xe2x80x9cManufacture of Superfine Organic Fibers,xe2x80x9d Apr. 15, 1954. Exxon Chemical developed a variety of commercial meltblowing dies, processes, and end-use products as evidenced by U.S. Pat. Nos. 3,650,866, 3,707,198, 3,755,527, 3,825379, 3,849,241, 3,947,537 and 3,978,185. Representative meltblowing patents of other companies (e.g. Beloit and Kimberly Clark) include U.S. Pat. Nos. 3,942,723, 4,100,324, and 4,526,733. More recent meltblowing die improvements are disclosed in U.S. Pat Nos. 4,818,463 and 4,889,476.
U.S. Pat. Nos. 5,145,689 and 5,236,641 each disclose a meltblowing die constructed in side-by-side units with each unit having separate polymer flow systems including internal valves.
As noted above, meltblowing is also used in the application of hot melt adhesives to substrates. Air assisted dies used in applying hot melt adhesives include mainly spiral dies and meltblowing dies. Spiral dies are disclosed in U.S. Pat. Nos. 4,785,996, and 4,983,109.
The meltblowing die of the present invention may be modular in structure, comprising a plurality of self-contained meltblowing modules. The modules are mounted in side-by-side relationship on a manifold so that the length of the die can be varied by merely adding modules to, or removing modules from, the structure. In a preferred embodiment, the modules are interchangeable and each includes an internal valve for controlling polymer flow therethrough.
The modular meltblowing die comprises a manifold and plurality of modules mounted on the manifold. The manifold has formed therein polymer flow passages for delivering a hot melt adhesive polymer to each module and hot air flow passages for delivering hot air to each module.
Each module includes a body, a die tip, and polymer and air flow passages for conducting hot melt adhesive and hot air from the manifold through each module.
In a preferred embodiment, the die tip of each module comprises (a) a triangular nosepiece terminating in an apex and polymer discharge means (i.e. fiber forming means) at the apex for discharging a row of closely spaced fibers, and (b) two rows of air passages flanking the row of fiber forming means. The fiber forming means may be in the form of an elongate slot or slots but preferably is in the form of a row of orifices. In either design a row of fibers are discharged from the die.
Hot air which flows through the manifold and each module is discharged as two rows of converging hot air streams at or near the apex. The polymer melt (such as hot melt adhesive) flows through the manifold and each module and discharges as a plurality of fibers into the converging air streams. The air streams contact and draw down the fibers depositing them as random fibers onto a collector or a substrate.
The air passages flanking the row orifices are shaped and positioned in relation thereto so that the discharging air streams contact opposite sides of the row of fibers and causes, at least some of the filaments, to flare out longitudinally in relation to the row of orifices. The pattern of fiber deposition on the substrate thus has a lateral dimension larger than the length of the row of orifices.
In a preferred embodiment, the air passages are in the form of air holes drilled in the die. The flanking air passages thus comprise two rows of converging air holes which lie in converging planes which intersect at or near the nosepiece apex. The converging planes define an included angle of between above 60xc2x0-90xc2x0. The air hole design eliminates the need for air plates commonly used in meltblowing dies and thus represents a significant improvement over conventional meltblowing die designs.
A particularly advantageous feature of the modular die construction of the present invention is that it offers a highly versatile meltblowing die. The die tip is the most expensive component of the die, requiring extremely accurate machining (a tolerance of 0.0005 to 0.001 inches on die tip dimensions is typical). The cost of long dies is extremely expensive (on the order of $1,300/inch). By employing the modules, which are relatively inexpensive ($300/inch), the length of the die can economically be extended to lengths of 200 or more inches. The air hole design permits controlled deposition of the fibers along the die length.
Another advantageous feature of the modular die construction is that it permits the repair or replacement of only the damaged or plugged portions of a die tip. With continuous die tips of prior art constructions, even those disclosed in U.S. Pat. No. 5,145,689, damage to or plugging of the die tip requires the complete replacement, or at least removal, of the die tip. With the present invention, only the damaged or plugged module needs replacement or removal which can be done quickly which results in reduced equipment and service costs. Another advantage of the preferred die constructed according to the present invention is as noted above, expensive and troublesome (e.g. plugging) air plates are not needed.
A still further advantage of the invention is the ability of the die to deposit the adhesive uniformly across on the substrate a plurality of modules. The outwardly flaring of the filaments permits the adhesive to deposit on the substrate in a lateral spacing, greater than the length of the row of orifices. With modular die tips, thus permits the orifice spacing on the die tip to be smaller than the spacing of prior art modular designs and still retain uniform properties across the length of the die. Also, the orifices at each end of the row of orifices receive more process air than those of the prior art designs.
As demonstrated in Example I, the die constructed according the present invention can be operated to deposit hot melt adhesive filaments onto a substrate in a sinusoidal pattern resembling a sewing stitch. There are many advantages of the sinusoidal pattern, including (1) less air consumption, (2) better edge control, (3) more defined patterns, and (4) forecastable bond strengths, among others.
In another embodiment of the die constructed according to the present invention, the air passages are positioned on the apex of the die adjacent the polymer passages instead of flanking the air passages. (The terms xe2x80x9cpolymer orificesxe2x80x9d, xe2x80x9cpolymer passagesxe2x80x9d and xe2x80x9cpolymer holesxe2x80x9d when referring to polymer filaments extruded from the die tip are used interchangeably herein.)
In this alternate embodiment, the sinusoidal pattern can be better controlled by positioning the air passages in relation to the polymer passages.
Although the principles involved in attaining the sinusoidal, stitch-like pattern of hot melt deposition are not fully understood, it has been demonstrated that this pattern can be achieved by all of the die designs disclosed herein. It is believed that the sinusoidal stitch-like pattern is caused by combination of the following forces: (a) viscous drag forces (in the direction of air flow), (b) the aerodynamic drag force caused by the non-parallel air flowing around the filaments, and (c) lift forces (in a direction normal to the air flow). These forces can be controlled to cause the filaments to oscillate in the cross or transverse direction in relation to substrate movement and cause the filaments to be deposited in a sinusoidal, stitch-like pattern.